An organic solar cell is a device that can directly convert solar energy into electric energy by applying a photovoltaic effect. A solar cell can be divided into an inorganic solar cell and an organic solar cell, depending on the materials forming a thin film. Typical solar cells are made through a p-n junction by doping crystalline silicon (Si), which is an inorganic semiconductor. Electrons and holes generated by light absorption diffuse to p-n junction points and move to an electrode accelerating by the electric field. The power conversion efficiency in this process is defined as the ratio of electric power given to external circuit and solar power entering the solar cell, and the efficiency have reached approximately 24% when measured under a currently standardized hypothetical solar irradiation condition. However, existing inorganic solar cells already have shown the limit in economic feasibility and material demands and supplies, therefore, an organic semiconductor solar cell have come into the spotlight as a long-term alternative energy source due to easy fabrication, a low cost, and various functionality.
For the solar cell, it is important to increase efficiency so as to output as much electric energy as possible from solar energy. In order to increase the efficiency of this solar cell, generating as many exitons as possible inside a semiconductor is important, however, it is also important to pull the generated charges to the outside without loss. One of the reasons for the charge loss is the dissipation of generated electrons and holes due to recombination. Various methods have been proposed to deliver generated electrons and holes to an electrode without loss, however, additional processes are required in most cases and therefore, manufacturing costs can increase.